1. Technical Field
The present invention relates to the light emitting field, and more especially, to a light emitting diode and its fabrication method.
2. Description of Related Art
Light Emitting Diode (LED), having the advantages of long service life, low energy consumption and others, is widely used in various fields. In particular, with the greatly improved lighting performance, LED is often used as a light emitting device in lighting field. Wherein, the group III-V compound semiconductors such as gallium nitride (GaN), has tremendous application potential in high-brightness blue LED, blue laser and other photoelectric devices due to its wide band gap, high light emitting efficiency, high electronic saturation drift velocity, stable chemical property and other characteristics, which has aroused wide attention.
However, those semiconductor LEDs in the prior art have the problem of low light emitting efficiency. For a conventional LED without packaging, the light extraction efficiency is generally several percent, because a large amount of energy gathers in the device and fails to give out, thus causing energy waste and also affecting the service life of the device. Therefore, to improve the light extraction efficiency of a semiconductor LED is of vital importance.
Based on the abovementioned application demands, many methods for increasing the light extraction efficiency of an LED are applied to the devices, for instance: surface roughening and metal reflector structures.
CN 1858918A discloses a GaN-based LED with an omnidirectional reflector structure and its fabrication method. According to FIG. 1, the LED comprises: a substrate 1, an omnidirectional reflector 4 grown on the substrate 1, and a GaN LED chip 13 fabricated on the omnidirectional reflector 4. The GaN LED chip 13 includes: a sapphire substrate 5, an N type GaN layer 6, an active region quantum well layer 7, a P type GaN layer 8, a P type electrode 9, a P type soldering pad 10, an N type electrode 11, and an N type soldering pad 12; wherein the omnidirectional reflector 4 grown on the substrate 1 is stacked by high refractive index layers 3 and low refractive index layers 2, the high refractive index layer 3 is in contact with the sapphire substrate 5, the low refractive index layer 2 is in contact with the substrate 1, the refractive index of the high refractive index layer nH>the refractive index of the low refractive index layer nL>the refractive index of the sapphire material n, and satisfies the formula of
                    sin                  -          1                    ⁢              n        nH              <                  tan                  -          1                    ⁢              nL        nH              ,wherein n, nH and nL represent refractive index. This patent forms an omnidirectional reflector structure on the bottom surface of the LED chip to reflect the light emitted by GaN material at a high refractive index upwards within the omnidirectional range so as to improve the light extraction efficiency of the LED. However, the fabrication process of the LED requires the forming of a film structure stacked by multiple high refractive index layers and low refractive index layers on the substrate, which is a complicated technique and the manufacturing cost is very high.